The present invention relates to a hazard detector, and more particularly, in one form to a fire-hazard detector that includes protection against incorrect installation, and/or for which in-situ testing is facilitated. In another form, the invention is applicable to a hazard detector the operation of which can be modified when it is in a test mode. The invention is applicable to detectors sensitive to other hazards, e.g. (without limitation) toxic gas, radiation or intruders. The term ‘hazard detector’ thus is to be construed accordingly.
Conventional fire detectors are normally used in simple two-wire circuits powered by a battery or other secure DC supply. When in a stand-by mode, such detectors present a high resistance between the two circuit wires and draw a negligible current from the battery, whereas in an alarm mode they introduce a low resistance across the two circuit wires. The high resistance presented during the stand-by mode normally makes it impossible during that mode to monitor the presence of such a detector on a two-wire circuit. Therefore, to ensure that such fire detectors will operate properly in the alarm mode, it becomes important to determine that they are correctly connected., and regular testing is required.
Some detectors are made insensitive to the polarity of the power supply so as to simplify their installation and avoid problems that occur when a polarity-sensitive device is installed improperly. One way to make a detector insensitive to power-supply polarity is to introduce a diode bridge; this is illustrated in FIG. 1. The drawback with this arrangement is two-fold; it adds cost, and it increases the minimum operating voltage of the detector significantly due to the voltage drop across the diode bridge.
If a diode bridge or another circuit is not introduced to make the detector insensitive to power-supply polarity, then it becomes necessary to protect the electronic circuit in the detector against a reverse-polarity connection in some other way. This is normally achieved by adding to the detector a diode in parallel with the electronic circuit of the detector and in reverse polarity across the power supply when the detector is properly connected; this is illustrated in FIG. 2. If the detector happens to be connected in a reverse fashion across the power supply, the diode will also be connected in the wrong direction, which will result in a short-circuit being presented to the control panel, indicating a wiring fault. While this arrangement may be acceptable for many control panels, there are some panels in which a momentary reversal of the power supply is used as part of a line-monitoring system; in such control panels, a short-circuit caused by polarity reversal is not acceptable.
An alternative method of protecting the electronic circuit of a detector against reverse polarity is the inclusion in the detector of a blocking diode in series with the other electronic circuitry of the detector; one embodiment of this is illustrated in FIG. 3. This method will operate on all known systems. However, it has the disadvantage that an inadvertent reverse connection will not result in a fault condition being shown at the control panel. To verify correct connection it is necessary to initiate an alarm condition in the detector, either by using smoke or other appropriate stimulus or by using a special test facility. This is inconvenient in that the alarm condition will be registered by the control panel, which may cause an audible alarm to sound or other action to be taken (such as an automatic call to a fire department).